1. Field of the Invention
This invention relates to acoustic transducer arrays, and more particularly, to a method for fabricating a backing layer for use with such an array to electrically connect the individual transducer elements of the array to respective circuit elements.
2. Background of the Invention
Ultrasonic imaging systems are widely used to produce images of internal structure of a specimen or target of interest. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that are caused to travel into the body. Echoes from the tissues are received by the acoustic transducer element or elements and are converted into electrical signals. A circuit element, such as a printed circuit board, flexible cable or semiconductor, receives the electrical signals. The electrical signals are amplified and used to form a cross-sectional image of the tissues. These imaging techniques provide a safe, non-invasive method of obtaining diagnostic images of the human body.
The acoustic transducer which radiates the ultrasonic pulses comprises a plurality of piezoelectric elements arranged in an array with a predetermined pitch. The array is generally one or two-dimensional. By reducing the pitch of the piezoelectric elements in the array, and increasing the number of elements, the resolution of the image can be increased. An operator of the imaging system can control the phase of the electronic pulses applied to the respective piezoelectric elements in order to vary the direction of the output ultrasonic wave beam or its focus. This way, the operator can "steer" the direction of the ultrasonic wave in order to illuminate desired portions of the specimen without needing to physically manipulate the position of the transducer.
When one of the piezoelectric elements is energized, acoustic waves are transmitted both from the front surface of the element facing the imaging target and the rear surface of the element. It is desirable that the acoustic energy from the rear surface be substantially attenuated so that the image resolution is not adversely affected. If not attenuated, the rearward travelling acoustic signals can reflect off the circuit element and return to the transducer surface, causing a degradation of the desired electrical signal.
To remedy this situation, a backing layer of an acoustically attenuating material is disposed between the piezoelectric elements and the circuit element to attenuate the undesired acoustic energy from the rear surface of the piezoelectric element. Ideally, this backing layer would have an acoustic impedance matched to the impedance of the piezoelectric elements so that a substantial portion of the acoustic energy at the rear surface of the piezoelectric element is coupled into the backing layer.
A problem with the use of a backing layer between the piezoelectric element and the circuit element is that of providing electrical interconnection between the particular piezoelectric elements and the associated circuit elements. The interconnection problem is more difficult for two-dimensional arrays of more than three rows and columns of piezoelectric elements, since the internal elements will not have an exposed edge that easily accommodates electrical connection. In such two-dimensional arrays, electrical interconnection between the individual piezoelectric elements and the electric circuit which receives and processes the electrical signals is generally made in the Z-axis direction perpendicular to the array. However, as the number of elements within the array increases, and the pitch between the elements decreases, it becomes increasingly difficult to fabricate this interconnection.
One approach to provide the interconnection through the backing layer is disclosed in U.S. Pat. No. 4,825,115 by Kawabe et al., entitled ULTRASONIC TRANSDUCER AND METHOD FOR FABRICATING THEREOF. Kawabe teaches the use of printed wiring boards bonded directly to the piezoelectric array transducer elements. A backing layer is then molded onto the array around the boards, which extend outward from the molded backing layer. While Kawabe discloses a reliable interconnection method, the wiring boards provide a surface for undesired reflection of acoustic wave energy within the backing layer, and thus mitigate some of the beneficial acoustic attenuating properties of the backing layer.
Another approach is to form the entire backing layer from a contiguous block of acoustic attenuating material, as disclosed in U.S. Pat. No. 5,267,221 by Miller et al., entitled BACKING FOR ACOUSTIC TRANSDUCER ARRAY. Since the contiguous backing layer is generally free of internal obstructions, such as the Kawabe wiring boards, the backing layer would provide improved overall acoustic attenuating ability. Nevertheless, fabrication of the contiguous backing layer requires that delicate electrical conductors be threaded entirely through the solid backing layer without breakage. In practice, this presents a rather difficult task to accomplish, especially given large matrix size acoustic arrays having relatively narrow pitch and high numbers of individual transducer elements. As a result, the contiguous construction backing layer is not generally conducive to certain large scale fabrication techniques despite its other clear advantages.
Therefore, a critical need exists for an improved method for fabricating a backing layer to provide electrical interconnection between elements of an acoustic transducer array and corresponding contacts of an electrical circuit element. Such a backing layer should provide for sufficient attenuation of the outputted acoustic energy from the rear surface of the piezoelectric element while avoiding internal reflections of such energy back to the transducer element. The fabrication method should also be cost effective and readily adaptable for large transducer arrays having high numbers of piezoelectric elements with relatively small pitch.